Microlithography is used for producing microstructured components, such as, for example, integrated circuits or liquid crystal displays (LCDs). The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. In this case, the image of a mask (also referred to as a reticle) illuminated by the illumination device is projected, by the projection lens, onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
A characterization of the structures on the mask is performed both in respect of present deviations of the respective structure on the mask from the intended position predefined by the design (so-called positioning error or “registration error”) and in respect of the line width of the structures (“critical dimension” (CD)).
For determining the positioning error, various methods are known in the prior art. By way of example, a “threshold-based” image evaluation can be applied to the structures of the aerial image, as is known from US 2012/0063666 A1. Alternatively, by use of a position measurement system, a first aerial image of a segment of the mask can be recorded and compared with a simulated second aerial image, whereupon the positioning error is then equated with the distance between the structures to be measured in the measured first aerial image and the simulated second aerial image.
One problem that occurs in practice, however, is that the measurement image is deformed or distorted on account of the properties of the optical system (that is to say that a coordinate grid is not exactly at right angles on the measurement image), whereas the simulated image as an ideal simulated grid does not have this property.
One known approach for taking account of the distortion consists in the latter being calibrated or “extracted computationally,” i.e., the distortion being determined metrologically by a targeted measurement with test structures in the image field. In this case, however, the further problem occurs that the distortion taken as a basis in such a calibration is dependent both on the pupil illumination specifically used within the imaging optical unit of the position measurement system and on the type of structure used for calibration. In so doing, here and in the following text, “pupil illumination” is understood to mean the intensity distribution obtained in a pupil plane within the imaging optical unit of the position measurement system, in which the imaging optical unit images light coming from the mask onto a detector unit.
The distortion underlying the above-described calibration is no longer exactly valid for any other possible structures, in which the resulting structure-dependent and illumination-dependent differences in the distortion on which the calibration is based are measurable in the sub-nanometer range and may be significant.
With regard to the prior art, reference is made for example to WO 2001/012265 A1, DE 10 2007 033 815 A1 and DE 10 2006 059 431 A1, US 2010/0104128 A1, DE 10 2007 033 815 A1 and also the publication M. Längle et al.: “Pattern placement metrology using PROVE high precision optics combined with advanced correction algorithms,” Proc. SPIE 8082, 80820J (2011).